This application is based on patent applications Hei.11-89626 and 11-271184 filed in Japan, the contents of which are hereby incorporated by references.
1. Field of the Invention
This invention relates to an impact actuator using piezoelectric device as a driving source and an equipment such as a camera or a binocular using the impact actuator.
2. Description of the Related Art
An impact actuator using a piezoelectric device as a driving source is conventionally known. In the impact actuator, a driven member is movably coupled with a rod shaped driving member in an axial direction of the driving member. A piezoelectric device is fixed on an end of the driving member in a manner so that a direction of polarization of the piezoelectric device coincides with an axial direction of the driving member.
FIG. 11 shows a schematic configuration of a conventional impact actuator. FIGS. 12 and 13 show examples of circuit diagram of a slow charging circuit in FIG. 11. FIG. 14 shows an example of a slow discharging circuit in FIG. 11.
As can be seen from FIG. 11, the conventional impact actuator 100 comprises a rod shaped driving member 101, a driven member 102, a laminated piezoelectric device 103 and a driving circuit 104. The driven member 102 is coupled with the driving member 101 by a predetermined friction force. When a force acting on the driven member 102 is larger than the friction force, the driven member 102 can be slid on the driving member 101 in an axial direction thereof. An object such as the taking lens to be driven is fixed on the driven member 102. The piezoelectric device 103 is fixed on a base end 101b of the driving member 101 in a manner so that a direction of polarization of the piezoelectric device 103 coincides with the axial direction of the driving member 101. A pair of electrodes 103a and 103b are formed on both ends of the piezoelectric device 103. The electrode 103b is grounded and the electrode 103a is connected to the driving circuit 104.
The driving circuit 104 comprises a forward driver 105, a backward driver 106 and a controller 107 for controlling the drivers 105 and 106. The forward driver 105 drives the actuator so as to move the driven member 102 toward an open end 101a of the driving member 101. Hereinafter, the direction toward the open end 101a of the driving member 101 is called xe2x80x9cforwardxe2x80x9d. The backward driver 106 drives the actuator so as to move the driven member 102 toward the base end 101b of the driving member 101. Hereinafter, the direction toward the base end 101b of the driving member 101 is called xe2x80x9cbackwardxe2x80x9d.
In the conventional impact actuator 100 relatively moves the driven member 102 with respect to the driving member 101 by utilizing the difference of the friction forces between the driving member 101 and the driven member 102 when the driving member 101 is vibrated by different speed. The friction force between the driven member 102 and the driving member 101 becomes smaller when the driving member 101 moves quickly, and it becomes larger when the driving member 101 moves slowly. Thus, the driven member 102 moves with the driving member only when the driving member 101 moves slowly. When the driving member 101 repeats the slow forward movement and the quick backward movement in the forward driving operation, the driven member 102 relatively moves forward (forward driving operation). When the driving member 101 repeats the quick forward movement and the slow backward movement in the backward driving operation, the driven member 102 relatively moves backward (backward driving operation).
The forward driver 105 comprises a slow charger 105a and a quick discharger 105b. The backward driver 106 comprises a quick charger 106a and a slow discharger 106b. The slow charger 105a and the quick charger 106a respectively apply a voltage of a power supply in the direction of polarization to the piezoelectric device 103 for expanding the piezoelectric device 103 in the axial direction of the driving member 101. The slow charger 105a is a constant current charging circuit shown in FIG. 12 or FIG. 13 for restricting a charging speed of the electric charge by controlling a value of a charging current.
FIG. 12 shows a constant current circuit configured by a pnp type transistor Tr1, a Zener diode ZD and resisters r1 to r3. The resisters r1 and r2 are biasing resisters of the transistor Tr1. The Zener diode ZD is connected in parallel with the resister r2 which is connected to a base of the transistor Tr1. By such a configuration, a base voltage of the transistor Tr1 is controlled at a fixed value by the Zener diode ZD, so that a voltage drop by the resister r1 can be stabilized. As a result, a collector current of the transistor Tr1 is controlled to be constant.
FIG. 13 shows another constant current circuit configured by a pnp type transistor Tr1, an npn type transistor Tr2 and resisters r1 and r3. A parallel circuit of the resistor r2 and the Zener diode ZD in FIG. 12 is replaced by the transistor Tr2. A base and a collector of the transistor Tr2 are respectively connected to an emitter and a base of the transistor Tr1. An emitter of the transistor Tr2 is connected to a power supply Vp. A base voltage of the transistor Tr1 is controlled to be a constant value by the transistor Tr2, so that a voltage drop by the resister r1 can be stabilized. As a result, a collector current of the transistor Tr1 is controlled to be constant.
The quick discharger 105b and the slow discharger 106b respectively apply a voltage in a reverse direction of polarization to the piezoelectric device 103 for contracting the piezoelectric device 103. The slow discharger 106b is a constant current discharging circuit shown in FIG. 14 for restricting a discharging speed of the electric charge by controlling a value of a discharging current.
FIG. 14 shows a constant current circuit configured by an npn type transistor Tr3, a Zener diode ZD and resisters r4 and r5. The resister r4 restricts the discharging current. A voltage of a base of the transistor Tr3 is controlled to be a predetermined value by the Zener diode ZD, so that a voltage drop by the resister r4 can be stabilized. As a result, an emitter current flowing the resister r4, which is the discharging current, is controlled to be constant.
The controller 107 controls the operations of the forward driver 105 and the backward driver 106. When the driven member 102 is to be moved forward, the controller 107 alternately drives the slow charger 105a and the quick discharger 105b. When the driven member 102 is to be moved backward, the controller 107 alternately drives the quick charging circuit 106a and the slow discharger 106b. 
When the slow charger 105a and the quick discharger 105b are alternately driven in a forward driving operation, the piezoelectric device 103 repeats slow expansion and quick contraction alternately. Thus, the driving member 101 repeats a slow forward movement and a quick backward movement. On the contrary, when the quick charger 106a and the slow discharger 106b are alternately driven in a backward driving operation, the piezoelectric device 103 repeats quick expansion and slow contraction alternately. Thus, the driving member 101 repeats a quick forward movement and a slow backward movement.
Accordingly, the driven member 102 relatively moves forward with respect to the driving member 101 in the forward driving operation, and the driven member 102 relatively moves backward with respect to the driving member 101 in the backward driving operation.
When the impact actuator is used as an actuator for moving an optical lens system of a handy equipment such as camera or a binocular, it is preferable that the driving circuit is simple and small with regard to lightening and downsizing of the equipment. The driving circuit 104 of the conventional impact actuator, however, is configured to restrict the charging current and the discharging current by the constant current circuits, so that it needs many elements constituting the driving circuit 104. Thus, it is difficult to lightening and downsizing the driving circuit 104.
An object of this invention is to provide an impact actuator and an equipment using the impact actuator which can be lightened and downsized.
An impact actuator in accordance with an aspect of this invention comprises a piezoelectric device, a driving member fixed on an end of the piezoelectric device in a direction of polarization of the piezoelectric device, a friction member engaged with the driving member by a friction force and causing a relative movement in an axial direction of the driving member, and a driving apparatus.
The driving apparatus comprises a first driver for applying a voltage in a direction of polarization of a piezoelectric device for charging and discharging electric charge to and from the piezoelectric device at a predetermined charging speed, a second driver for applying a voltage in a reverse direction of polarization of the piezoelectric device for charging and discharging electric charge to and from the piezoelectric device at the substantially the same charging speed as that of the first driver, and a controller for controlling the first driver and the second driver alternatively in different driving time periods.
The friction member can be moved relative to the driving member under a predetermined condition when the piezoelectric device of the impact actuator is driven by providing a relatively large difference between the charging time for charging the piezoelectric device in the direction of polarization and the charging time in the reverse direction of polarization.
The above-mentioned configuration of the driving apparatus needs no current restricting element such as a resistor for restricting charge or discharge speed of electric charges, so that the configuration of the driving apparatus becomes very simple. Consequently, the impact actuator and an equipment using the impact actuator can be lightened and downsized.